This invention relates in general to semiconductor wafer cleaning, and more specifically to processes and apparatus for dissolving ozone in deionized rinse water for cleaning and passivating a semiconductor wafer.
Once semiconductor wafers have been cleaned, it is important to provide a thin native oxide layer on the wafers as soon as possible in order to prevent contamination of the wafer during its fabrication. The process of forming this layer is termed surface passivation.
Native oxide readily forms on bare silicon wafer surfaces with or without ozone. When it is formed slowly or in an uncontrolled manner it will tend to incorporate high levels of SiOx particles or other contaminants. In prior art techniques, surface passivation is provided by hydrogen peroxide. It has been found, however, that hydrogen peroxide often contains undesirable metallic contaminants, reducing wafer yield or requiring additional cleaning steps.
Further prior art techniques expose the bare silicon wafer surface to high levels of ozone, forming a quick and clean native oxide on the surface. Such a native oxide layer can be provided by subjecting the wafers to a bath of ozone-rich water. In order to achieve a rapid silicon surface conversion to a native oxide, the ratio of ozone to water should be more than 7 parts per million.
Current techniques for ozonating water are, however, inadequate. Ozone quickly leaves the water bath and so the wafers do not receive the desired native oxide layer. Furthermore, the existing techniques for generating the ozone and ozonating the water are expensive, complex, and power-hungry.
The invention provides a simple, inexpensive, ozone-generating and ozone-capturing apparatus and method for semiconductor wafer cleaning, which require less power than existing methods. The ozone generation apparatus and method uses ultraviolet light at a range of wavelengths concentrated at 185 nm to convert gaseous oxygen (O2) to gaseous ozone (O3). Oxygen gas is pumped into an optically-opaque chamber containing a short-UV high-intensity lamp. The lamp""s ultraviolet light ionizes the molecular oxygen, converting it to ozone. The ozone is then pumped out of the chamber. The ozone-capturing apparatus and method uses a reverse polytetrafluroethylene (Teflon) filter. The filter is immersed in a housing of water. Ozone is pumped into the filter under pressure with a check valve to prevent the back flow of ozone. The receiving housing is filled with water and is likewise sealed. The ozone under pressure is forced out of the Teflon filter and into the surrounding water. The ozonated water is withdrawn from the base of the housing and is passed to a wafer ozone bath for applying the ozonated water to one or more semiconductor wafers. The ozonated water quickly produces on the wafers a thin layer of virtually contaminant free native oxide. This layer of native oxide aids in protecting the wafers from further contaminants during the further wafer processing. Note that native oxide is self-limiting in its growth, with the final thickness (usually less than 50 xc3x85) dependent on the ambient temperature and pressure under which it is formed.